Whole mesh rotates with armature, not just the arms
If you use automatic weight, Blender can only &quot;take&quot; what it has. Since there is only one bone in your armature, Blender takes all vertices to do it, so your whole person moves.
If you add bones like this:

it works:

blend file (but be aware, i just made &quot;rough&quot; bones, and just on one side to prove it works):